1. Field of the Invention
The present invention relates to a method of modifying a surface of a polymeric nanofiber, for example, a polymeric nanofiber mat, by modified plasma treatment, and a polymeric nanofiber having a structurally nanopatterned surface obtained using the method.
2. Discussion of Related Art
In recent years, as nano-imprinting technology have attracted much attention as one of next-generation lithographic processes, lots of research institutions and organizations have paid attention have taken active interests in fabrication and application of nanopatterns using such technology. With the development of nano-patterning technology, it is possible to manufacture a substrate having a nanopattern, or a micro-nano hybrid pattern. In this case, representative patterning technology used to manufacture the substrate includes soft lithography, UV lithography, plasma lithography, thermal lithography, etc. Particularly, plasma lithography is one of patterning methods which have paid current attention since they can be used to physically and chemically surface-modify a surface of the substrate.
However, conventional polycaprolactone (hereinafter abbreviated as ‘PCL’) microfiber patterning using plasma may be performed to provide nano-sized roughness and hydrophilic properties. However, the conventional PCL microfiber patterning has a problem in that it is difficult to pattern PCL into a certain nano-sized morphology due to the high working temperature upon plasma treatment since PCL is melted at a certain temperature. Further, PCL polymers widely used as biopolymers has a limitation in that they are sensitive to the temperature, and thus materials may be melted to collapse a fiber morphology when high power is applied to enhance plasma treatment efficiency (see Yan D. et al., J. Biomed. Mater. Res. Part A 2013, 101, 963-72; Nandakumar A. et al., Biofabrication, 2013, 5, 015006-015020).
Also, when plasma treatment is performed at low power to avoid a high working temperature upon patterning, plasma treatment efficiency may be degraded, which makes it difficult to effectively perform the patterning [Nandakumar A. et al., Biofabrication, 2013, 5, 015006-015020].
Meanwhile, in the tissue engineering, biomedical scaffolds requires various physical and biological properties, as follows: (1) supporting a structure to induce attachment, proliferation, and differentiation of seed cells, (2) mechanical properties substantially similar to innate tissue surrounding the scaffolds, (3) a physical clue, for example, topography inducing cell attachment to a binding site, (4) a mechanism for transferring a growth factor, and (5) a porous microstructure enabling diffusion of nutrients and exchange of metabolites inducing angiogenesis.
Various synthetic materials used to fabricate a scaffold for tissue regeneration, which has the above- described properties, have been proposed. By way of example, the synthetic materials may include polycaprolactone (PCL), poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly(lactic acid-co-glycolic acid) (PLGA), and the like, and natural materials may, for example, include collagen, alginic acid, silk fibroin, chitosan, gelatin, and the like. Among such biocompatible materials, PCL has been widely researched for wide applicability in hard tissue regeneration due to biocompatibility, slow biodegradability, structural solidity, controllable mechanical properties, and processability.
However, PCL has low bioactivity due to the lack of hydrophobic bio-functional groups on a surface thereof. As a result, the growth rate of tissues is delayed by lowering an initial cell affinity of PCL and reducing cellular interactions.
For these reasons, a surface modification of PCL has been used as a tool for improving bioactive properties of synthetic PCL scaffolds. Typical surface modification methods may include chemical treatment, laser treatment, ion beam irradiation, and plasma treatment.
Among these methods, plasma treatment has excellent probability since it does not have an influence on bulky mechanical properties of the materials.
Further, in plasma treatment, a harmful toxic solvent that may remain on a surface of a material and cause damage to seed cells is not used. Also, a variety of gases used for plasma discharge may act as cell binding sites on a modified PCL surface. Therefore, the plasma treatment is one of the most attractive processes in the field of bio-fabrication.
Habibovic, et al. reported that, when electrospun fibers made of a poly(ethylene terephthalate)/poly(butylene terephtalate)(PET/PBT) copolymer are exposed to radio-frequency oxygen plasma, plasma-treated scaffolds have a positive effect on differentiation of osteoblasts in human mesenchymal stromal cells (hMSCs) (see A. Nandakumar, Z. T. Birgani, D. Santos, A. Mentink, N. Auffermann, K. van derWerf, M. Bennink, L. Moroni, C. van Blitterswijk and P. Habibovic, Biofabrication, 2013, 5, 015006j.).
Another research by Sun, et al. reported that rapid-prototyped PCL is treated with plasma for different periods of time, and significant differentiation of osteoblasts, secretion of osteocalcin proteins, and calcium mineralization are observed on a surface of PCL after 3 minutes of plasma treatment, as evaluated by alkaline phosphatase (ALP) activities (see E. D. Yildirim, D. Pappas, S. Guceri and W. Sun, Plasma Processes Polym., 2011, 8, 256.). Such results indicate that the use of optimized plasma exposure time may cause differentiation of 7F2 mouse osteoblasts.
As pointed out by Habibovic, et al., however, the plasma treatment for surface modification is effective, sample and inexpensive, but has some problems. In particular, the plasma treatment has a problem in that it is difficult to control the size of a surface pattern of PCL with high resolution.
The conventional technology of modifying a surface of a PCL microfiber using plasma is useful in giving low roughness and hydrophilicity, but has problems in that materials are restrictive due to a high working temperature, and desired topography and roughness may not be obtained due to a decrease in plasma treatment efficiency when a process is performed at low power.